Daft thought, but by any chance are you getting the music into the system via some sort of
lead plugged into the headphone socket on a portable music player?

If so, does
this lead look something like a 3.5mm mini jack to a single 1/4 inch jack plugged into a
line input?

That does NOT work and will sound VERY thin, because the line
input socket is usually not stereo but balanced and that results in the system producing
only the difference between the two stereo channels.

If pulling the input jack
out slightly makes things come right (possibly in mono, but better) then you just need a
different input lead that is wired correctly for this application.

Regards,
Dan.

--------------------Audiophiles use phono leads because they are unbalanced people!